CAT scanners of the third-generation type include an X-ray source and X-ray detector system secured respectively on diametrically opposite sides of an annular disk. The latter is rotatably mounted within a gantry support so that during a scan the disk rotates about a rotation axis while X-rays pass from the source through an object positioned within the opening of the disk to the detector system.
The detector system typically includes an array of detectors disposed as a single row along an arc of a circle having a center of curvature at the point, referred to as the "focal spot," from which the radiation emanates from the X-ray source. The X-ray source and array of detectors are all positioned so that the X-ray paths between the source and each detector all lie in the same plane (hereinafter the "rotation plane" or "scanning plane") normal to the rotation axis of the disk. Because the ray paths originate from substantially a point source and extend at different angles to the detectors, the ray paths resemble a fan, and thus the term "fan beam" is frequently used to describe all of the ray paths at any one instant of time. The X-rays that are detected by a single detector at a measuring instant during a scan are hereinafter referred to as a "ray." The ray is partially attenuated by the mass of all objects in its path so as to generate a single intensity measurement as a function of the attenuation, and thus the density, of the mass in that path. Projections, i.e., the X-ray intensity measurements, are typically done at each of a plurality of angular positions of the disk.
An image reconstructed from data acquired at all of the projection angles during the scan will be a slice along the scanning plane through the object being scanned. In order to reconstruct a density image of the section in a defined rotation plane, the image is typically reconstructed in a pixel array, wherein each pixel in the array is attributed a value calculated from the attenuation of all of the rays that pass through it during a scan. As the source and detectors rotate around the object, rays penetrate the object from different directions, or projection angles, passing through different combinations of pixel locations. The density distribution of the object in the slice plane is mathematically generated from these measurements, and the brightness value of each pixel is set to represent that distribution. The result is an array of pixels of differing values which represents a density image of the scanning plane.
In order for the image reconstruction process to work, the position of the rays must be precisely known. In order to accurately position the rays without an unmanageable amount of calibration and correction, it is therefore very useful to have accurately located detectors, and measurements accurately timed so that the angular position of each detector for each projection is predetermined.
Further, since dense matter tends to scatter X-rays, it is important that any radiation that does not traverse a straight line from the source to each detector be excluded from the measurements by each such detector. To remove this scattered radiation, a series of very thin anti-scatter plates is typically inserted between the detectors and the object with the individual plates aligned so as to collimate the rays from the radiation source by allowing to pass to the detectors substantially only those rays traversing a straight, radial line between the source and each detector.
Unfortunately, the need for the anti-scatter plates creates additional difficulties because if they cast an X-ray "shadow" on a detector, they will interfere with its measurements. Not only will the output of each shadowed detector be reduced, but it will also be modulated by the least vibration or lateral movement of the source, anti-scatter plates and/or detectors.
The difficulty of meeting these requirements becomes evident when one considers that in order to provide the kind of resolution expected of modern X-ray tomographic scanners, the detectors number in the hundreds with several detectors located within a single degree of the fan beam arc. This makes the width of a typical detector on the order of a millimeter, and the dimensions of a typical anti-scatter plate about 20 mm long in the radial direction by about 0.1 mm thick, requiring extremely accurate detector and anti-scatter plate location and alignment. To further compound the problem, the whole assembly is usually rotated around the scanned object at a rate of about 60 to 120 rpm, generating substantial varying forces and requiring rugged mounting techniques.
Previous attempts to satisfy these difficult requirements have produced machines of very large mass, requiring very costly, painstaking assembly techniques with a great deal of effort spent in alignment of the anti-scatter plates and detectors. If for any reason one or more elements has to be replaced or realigned, the reassembly and realignment process is usually too demanding to be performed in the field, and the entire detector subsystem often has to be returned to the factory.
One approach to this problem is to establish preassembled modules for the detector and anti-scatter plate arrays, as disclosed in, for example, U.S. Pat. No. 5,487,098 to Dobbs et al., assigned to the assignee of the present invention. The detector and anti-scatter plate modules must each be attached to a support structure or spine which must then be attached to a rotating gantry of the tomography system. Each detector module therefore must be aligned with a corresponding anti-scatter plate module, and each pair of modules must be aligned relative to the focal spot in order to maximize receipt of radiation.
Another approach is disclosed in U.S. Pat. No. 4,338,521 to Shaw et al., in which a modular detector array includes two detachably assembled portions, one containing the detectors and the other containing the anti-scatter plates. The two portions of the array must be assembled together in order to establish their mutual alignment. The assembled module must then be aligned with the radiation source and then fixedly mounted to the tomography apparatus.
A difficulty with high-resolution detector subsystems is obtaining and maintaining the relatively tight alignment requirements of the detectors and anti-scatter plates with the x-ray beams from the radiation source. Tolerances are further strained by any temperature and vibrational changes in the relative alignment of the subsystems.